The present invention relates to a digital transmission control equipment, more precisely relates to a digital transmission control equipment for interchanging digital data, which receives coded digital signals, and corrects waveforms when the coded signals received are transmitted to other units.
A block diagram of a conventional digital transmission control equipment is shown in FIG. 7.
The conventional equipment has a waveform shaping circuit 50, a timing circuit 52 and a waveform regenerating circuit 54.
The wave shaping circuit 50 reforms waveforms of coded digital signals received into shaped digital signals which can be processed in subsequent circuits.
The timing circuit 52 generates receiving clock pulses by Phase Lock Loop (PLL) technology. The timing circuit 52 has a phase comparator 56, a low-pass filter 58, an amplifier 60 and a voltage control oscillator (VCO) 62. The phase comparator 56 compares the phase of the shaped digital signals with that of the output signals of the VCO 62, and outputs an average direct current (DC) voltage which is in proportion to the phase difference therebetween. The low-pass filter 58 removes high frequency noise from the average DC voltage output from the phase comparator 56. The amplifier 60 amplifies the average DC voltage output from the low-pass filter 58 to a prescribed input level of the VCO 62. A cycle time of the central oscillating frequency of the VCO 62 is approximately equal to a cycle time of the coded digital signals.
In case of no input, the VCO 62 runs freely, and adding synchronous signals for detecting phase to the head of the coded digital signals is required to coincide the frequency of the VCO 62 with that of the coded digital signals.
The waveform regenerating circuit 54 synchronizes the shaped digital signals from the waveform shaping circuit 50 with the receiving clock pulses detected by the timing circuit 52, and outputs the shaped digital signals.
Next, the action of the conventional equipment will be explained.
When coded digital signals, including synchronous signals whose level and duty have been disturbed, reach the waveform shaping circuit 50, shaping of the coded digital signals (such as correcting timing of leading and/or trailing edges of the waveform and voltage level) is executed in the waveform shaping circuit 50. The shaped digital signals so shaped by the waveform shaping circuit 50 are input to the timing circuit 52 and the waveform regenerating circuit 54.
In the timing circuit 52, the phase of the synchronous signals added to the shaped digital signals is compared with that of the output of the VCO 62 by the phase comparator 56, and the average DC voltage corresponding to the phase difference therebetween is output. High frequency noise in the average DC voltage is removed by the low-pass filter 58. Then the average DC voltage is amplified by the amplifier 60 and is input to the VCO 62. The oscillating frequency of the VCO 62 is controlled by the average DC voltage input, and the phase of the output (oscillating waves) of the VCO 62 is compared with that of the synchronous signals again. The above described steps are repeated until the frequency of the receiving clock pulses (the output of the VCO 62) equals the frequency of the synchronous signals.
In the waveform shaping circuit 50, the shaped digital signals and the receiving clock pulses are processed so as to generate coded digital signals synchronized with the receiving clock pulses. The coded digital signals so synchronized are sent to transmission routes.
The conventional digital transmission control equipment has, however, the following disadvantage:
Since the PLL circuit is included in the timing circuit 52, the synchronous signals for synchronizing the frequency of the VCO in the PLL with that of the coded digital signals must be added the head of the coded digital signals. Therefore, it takes a relatively long time to change the frequency of the VCO to that of the synchronous signals.
Because of this disadvantage, in the case of, for example, an optical communication system which must have a plurality of control equipment provided at regular intervals, or in another case of many control equipment provided in a transmission route, so much time is spent in each control equipment that transmission efficiency is lowered.